Nocturnal behavior of animals was taking place long before the first mammals walked the face of the Earth, according to new research.

Mammals are mostly nocturnal species, and many others are active during twilight hours. Biologists largely believed that this behavior first developed about 200 million years ago, during the rise of the first mammals to walk the face of the Earth. This theory was based on the relatively large brain size of animals, useful for processing smells, touch and hearing - senses vital for living and hunting in low-light conditions. Light-sensitive chemicals found in eyes of mammals also provided evidence for placing the start of nocturnal behavior with the rise of our class of animals.

Synapsids, a group of ancient herbivores that gave rise to all mammals, may have also been active at night, according to researchers.

"Synapsids are most common in the fossil record between about 315 million years ago and 200 million years ago. The conventional wisdom has always been that they were active during the day (or diurnal), but we never had hard evidence to say that this was definitely the case," Kenneth Angielczyk, a curator at The Field Museum, said.

Nocturnality in synapsids could predate mammals by more than 100 million years, the new study reveals.

Scleral ossicles, tiny bones found in the eyes of some birds, lizards and other vertebrates were examined, in order to date the development of nocturnal behavior. The tiny bones surrounding the irises of these animals are not present in modern mammals, but were found in Synapsids.

"... [T]his information allows us to make predictions about the light sensitivity of the eye, which usually reflects the time of day an animal is active," Lars Schmitz, professor of biology at Claremont McKenna, Pitzer, and Scripps Colleges, told the press.

The delicate ring of bones is only rarely preserved in Scleral ossicles, so they have rarely been used to investigate the development of nocturnality. Angielczyk and Schmitz examined specimens of synapsids in museums and other collections, gathering information about scleral ossicles in 24 separate species. This information was compared to the body parts in birds and lizards having known day/night patterns.

Eyes of synapsides were found to be sensitive to a wide range of light conditions, including low-light conditions. This suggests the ancient animals were likely active in twilight and at night.

Study of nocturnal behavior in Synapsids and what it could mean for the evolution of mammals was profiled in the journal Proceedings of the Royal Society B.